The invention relates to a method for pressing hollow blanks from glass between a mold having an inner wall complementary to the outer side of a blank and a plunger comprising an open-pore plunger head having a surface complementary to the inner side of the blank and an inner cavity which is filled with liquid of which in each working cycle a partial amount evaporates by heat transfer from the glass to the plunger head at the surface of the latter, forms between said surface and the glass a separating vapour layer and is replaced by further liquid introduced into the plunger head.
For making lightweight glass containers the press-and-blow method has established itself. In this method, a parison, in contrast to the glass blowing method, is not made by blowing but by pressing. The pressed parison is subsequently blown up in a blowing mold.
With this method, wide mouth glasses such as jam glasses have been made for decades. By improving the technology it is possible with this method to make narrow-neck containers such as bottles which are distinguished by a particularly uniform wall thickness distribution and low weight. Difficulties are encountered in the pressing of the parison when the operation must be carried out with a slim plunger (press ram) hardly greater than finger thick having a flank angle of for example only 2.degree. to 3.degree..
Hitherto, when using solid plungers it was only by particularly accurate control of the working times and operating temperatures thereof that shrinkage of the glass onto the plunger on cooling thereof could be avoided. Another disadvantage when using solid plungers is that due to high shearing stresses in the glass during the pressing operation and due to sliding processes between the glass and the surface of the plunger it is not possible to prevent completely damage to the glass surface and metal abrasion of the plunger. Damage to the glass surface and small metal particles embedding themselves in the glass reduce the utility strength of the glass container, in particular its impact strength.
To avoid these disadvantages, U.S. Pat. No. 4,682,996 ("Foster") has suggested a method of the type described at the beginning and an apparatus for carrying out said method. The thin vapour layer forming between the glass and the surface of the plunger is intended to act as a lubricating film facilitating sliding between the glass and the plunger and thus avoiding metal abrasion from the plunger and reducing the heat transfer between the glass and the plunger. However, hitherto it was not possible to achieve this objective with adequate certainty in particular with slim plungers as are necessary for making narrow neck containers.